This invention generally relates to methods and apparatus for testing optical systems and particularly to devices and techniques for the automated measurement of a variety of parameters of optical surfaces and/or elements including radii of curvature, surface shape, thickness, power, focal length, wavefront, topography, and aberrations.
Throughout the process for fabricating optical systems or components, it is frequently necessary to determine if, and how well, optical surfaces or elements conform to their designers stated requirements. Not only does the performance of optical systems in final form need to be verified, but various parameters of their components need to undergo intermediate testing for conformance with their specifications prior to final assembly as a system. Indeed, even the tools of fabrication, especially molds for the formation of plastic or glass lens elements, need to be tested for compliance with design specifications. Some of the most frequently encountered measurements that need to be made are radius of curvature of surfaces in either convex or concave form, surface topography, thickness, power, and various focal lengths.
Classically, radius of curvature is measured through the use of a hand-held instrument called a spherometer, which measures the sagittal height (sag) of the surface over a known diameter and then displays the radius of curvature on a dial or other visual display after an internal calculation that relates radius to sag height and the known diameter. However, the accuracy of such devices are prone to relatively large errors because sag heights are usually small dimensions that are difficult to accurately measure mechanically.
A more accurate technique for radii measurement involves the use of an auto-collimating microscope in an arrangement referred to as a radiusscope. Here, one first focuses on the surface to be measured and then on the center of curvature of the surface where a reticle image has been imaged back on itself by reflection from the test surface. The positions of the microscope are recorded, and the difference between them represents the radius of curvature to limits of accuracy, which depend on the preciseness of the length measurements and the ability of the operator to accurately focus.
Where the spherometer suffers from problems of precision, the use of the radiusscope, which can be accurate to microns if care is taken, is time consuming and dependent on operator skill and experience.
The thickness of an optical element is more or less important depending on its assigned role in a particular design and can be critical where the design relies heavily on its precision for aberration control or the like. Thickness obviously can be measured directly by mechanical means, which may also be automated, but there is always the danger of damaging part surfaces with mechanical approaches.
Power and focal length are always of interest and can be calculated from classical lens makers formulae having knowledge of the various numerical values required as, for example, index of refraction, radii, and thickness.
For rapid qualitative tests other approaches are also often employed. For this purpose, reliance on the Foucault knife-edge test is often made, and while the Foucault test has its uses, it suffers from an inability to sense changes in slope deviation that are changing only slightly in magnitude or direction. For more accurate quantitative results, reliance is made on interferometric and screen methods, such as the Hartmann test, to perform wavefront analysis from which more gross properties may be derived. However, such tests, while informative, have been laborious and time consuming.
While those skilled in the art have provided a variety of ways for measuring many of the foregoing properties of optical elements and systems, there remains a need for an instrument for rapidly and accurately measuring a number of optical properties virtually simultaneously, and it is a primary object of the present invention to provide such a device.
Another object of the present invention is to provide methods and associated devices for automatically performing wavefront analysis, measuring surface topography, and determining radius of curvature, thickness, power and focal length of optical surfaces and/or elements with minimal dependence on operator skill.
Yet another object of the present invention is to provide an automated instrument for providing statistical analysis of quality in high volume production settings.
Another object of the present invention is to provide apparatus and methods for optically measuring the physical characteristics of surfaces that are at least partially reflective.
Still another object of the present invention is to satisfy all of the foregoing objects with a user-friendly device that is simple in its implementation and low in cost.
Other objects of the invention will in part be obvious and will in part appear hereinafter. A full understanding of the invention will best be had from reading the detailed description to follow in connection with the detailed drawings.
The invention comprises a system and method for automatically performing dynamic screen testing on a surface and determining its shape from which other optical parameters of interest may be derived and reported. A measuring head, consisting of a source, beamsplitter, objective lens, and lens array with a CCD camera, is mounted on a translation stage and moves along the optic axis of the head relative to the part under test. The part under test is mounted on an appropriate support, such as a three-point support nest, that automatically centers spherical parts on the optical axis of the system.
Light is projected along the optical axis through a microscope objective or other appropriate lens to illuminate the part under test with a predetermined wavefront, preferably spherical, so that subsequent calculations are made simpler if this light is recollimated parallel to the optical axis of the system. Light reflected from the part under test passes back through the lens, after which it passes through a pellicle or cube beamsplitter towards a CCD camera. A two-dimensional array, preferably in the form of a pair of crossed lenticular screens, is placed in front of the CCD active area so that a series of sharp images are formed on the CCD array. When the system measuring head is positioned so that the focal point of the objective is located near the surface of the part under test or near its center of curvature, then the incoming nearly parallel light produces a series of spots on the CCD active area. The shifts in the pattern of spots are used to determine the shape of the surface under test. Mathematical analysis of this shape provides information on the radius of curvature of the part (if spherical), the xe2x80x9cSphericalxe2x80x9d and xe2x80x9cCylindricalxe2x80x9d radii of curvature of a toric part (along with the angle between the major axes and a given reference axis), and the xe2x80x9cShape Factorxe2x80x9d of an aspheric part. For ease of interpretation, the overall shape can be expressed in various ways, including Zernike polynomials. Software performs this analysis and facilitates providing results in many useful formsxe2x80x94contour plots, wire-frame models of deviation, direct readout of coefficients, direct readout of RMS surface form, direct readout of peak-to-valley difference, etc. Display screens are customizable for the engineering specialist or on-the-floor auditing and measurement for production. Custom processing capabilities are available using Visual Basic(copyright) and an Object Linking And Embedding (OLE(copyright)) interface.